The present invention generally relates to a semiconductor memory devices, and more particularly to a semiconductor memory device having a voltage generating circuit for precisely controlling the generation of negative voltages in order to prevent malfunctions associated with a parasitic diode in a wordline driver.
In semiconductor memory devices, especially, in DRAM devices which often times include huge numbers of memory cells for storing data, malfunctions are unwanted. Each memory cell includes a cell capacitor and a cell transistor controlling read/write operations of the inputted or outputted data. Accordingly, DRAM devices perform huge operations of data write, data read, and data refresh operations for the plurality of memory cell. Among others, the refresh operation is prone to be adversely affected by a leakage current. A large number of efforts have been undertaken to prevent or at least minimize the occurrence of the unwanted generation of the leakage currents from the cell.
The leakage currents can be classified into either a junction leakage current or a channel leakage current.
The junction leakage current is generated by defects at a junction boundary of the cell transistor. Channel leakage current is a leakage current flowing through a channel of the cell transistor.
The junction leakage current can be suppressed by reducing the ion concentration of the channel. However, the reduction in the ion concentration of the channel can result in increasing the channel leakage current. The channel leakage current can be reduced by increasing threshold voltage of the cell transistor. However this increase of the threshold voltage of the cell transistor can result in causing an increase in the junction leakage current.
In order to reduce such leakage currents, a negative wordline driving manner can be used to provide high voltage VPP when enabling a wordline, and to provide negative voltage VBBW lower than a ground level when disabling a wordline. As the negative voltage drops below than the ground level, there is voltage reversely biasing at the substrate. This voltage reversely biasing of the substrate is commonly referred to as backbias voltage VBB, in order to distinguish it from the negative voltage VBBW.
If such a negative wordline driving manner is used, refresh characteristics are improved and characteristics of other AC parameters are improved. In particular, a refresh time can be reduced, a VPP burden can be reduced when low level operation voltage Vcc is used, and a tWR (Write Recovery Time) can be improved so that the negative wordline driving manner has been widely used.
FIG. 1 is a block diagram of a voltage generating circuit generating negative voltage WBBW.
FIG. 1 shows a voltage generating circuit which includes a VBBW level detector 10, an oscillator 20, and a pumping unit 30.
The VBBW level detector 10 is shown having a fedback negative voltage VBBW for detecting the level thereof and outputting an oscillator enable signal OSCEN. The oscillator 20 is shown receiving the oscillator enable signal OSCEN to enable the oscillator 20 to generate a pulse signal OSC. The pumping unit 30 is shown having a capacitor and two diodes sharing a common node B in which the pumping unit 30 receives the pulse signal OSC to perform a charge pumping of the negative voltage.
The conventional negative voltage generating circuit generates regulated negative voltage using a negative feedback operation. When the negative voltage VBBW increases, the level detector 10 enables the oscillator enable signal OSCEN to enable the oscillator 20. As a result the negative voltage VBBW level is gradually reduced by the charge pumping until the oscillator 20 is disabled.
The VBBW level detector 10 can be configured as shown in FIG. 2. The VBBW level detector 10 of FIG. 2 includes PMOS transistors M1 and M2, and inverters INV1 and INV2. And, VNN means negative voltage VBBW to be fedback.
FIG. 2 shows that when the negative voltage VBBW increases, a source-drain equivalent resistance of the PMOS transistor M2 increases and as a result the voltage at node A rises. When the voltage at the node A reaches a trip point of the inverter, the output signal OSCEN rises to a high level to enable the oscillator 20 and the enabled oscillator 20 drives the pumping unit 30.
As shown in FIG. 3, the oscillator 30 includes inverters INV3, INV4, INV5, and INV6. The inverter INV6 can be implemented using a 3-phase inverter. The oscillator 20 can be implemented in any number of different configurations different from that of FIG. 3.
Referring back to FIG. 1, the pumping unit 30 can include a capacitor C and two diodes D1 and D2. When the pulse signal OSC output from the oscillator 20 is high, a node B is clamped into threshold voltage Vth higher than ground voltage by the diode D1, and the capacitor C is charged with positive voltage VDD. When the pulse signal OSC is low, the capacitor C supplies negative charges through the diode D2.
FIG. 4 shows a waveform diagram of the negative voltage VBBW generated as above. Referring to FIG. 4, it can be appreciated that the conventional negative voltage VBBW has a ripple having a relatively large height difference.
The negative voltage generated as above is biased into negative voltage of a wordline. The backbias voltage VBB provided to the wordline is generated in the same method to be biased into the word line.
FIG. 5 shows a general sub wordline driver transferring a signal of a main wordline MWLB to a sub wordline SWL connected to gates of a plurality of cells with sub wordline driving voltage FX and the negative voltage VBBW.
The sub wordline driver includes a sub wordline driving voltage unit and a negative voltage driver. By way of example, the sub wordline driver configured as a CMOS type. This sub wordline driver includes one PMOS transistor M3 and two NMOS transistors M4, M5, as shown in FIG. 5. The PMOS transistor M3 corresponds to the sub wordline driving voltage unit. The NMOS transistors M4, M5 correspond to the negative voltage driver. Herein, the number of the NMOS transistors corresponding to the negative voltage driver may be designed in a proper number considering drivability.
One terminal of the PMOS transistor M3 is connected to sub wordline driving voltage FX and the other terminal thereof is connected to a sub wordline SWL. The gate of PMOS transistor M3 is connected to an inverted main wordline MWLB.
One terminal of the NMOS transistor M4 is commonly connected to the sub wordline SWL and to one terminal of the PMOS transistor M3. The other terminal of the NMOS transistor M4 is biased with the negative voltage VBBW. The gate of the NMOS transistor M4 is connected to the inverted main wordline MWLB as in the PMOS transistor M3. One terminal of the NMOS transistor M5 is connected to the sub wordline SWL and the other terminal thereof is biased with the negative voltage VBBW. The gate of the NMOS transistor M5 is connected to the sub wordline driving voltage FX.
The backbias voltage VBB is applied as the substrate voltage of the NMOS transistors M4, M5.
In this configuration, parasitic diodes D3, D4 are formed between the substrate (P-type impurity) and the source (N-type impurity) of the NMOS transistors M4, M5. When the ripple of the backbias voltage VBB is applied to the substrate and the negative voltage VBBW is applied to the source is large, this voltage difference may result in being higher than the threshold voltage of the parasitic diodes. As a result when the parasitic diodes D3, D4 are turned on, undesired leakage current can occur which causes the semiconductor device to malfunction. Referring to FIG. 6, it can be appreciated that a potential difference between the backbias voltage VBB and the negative voltage VBBW may be equal to or greater than the Von.
Therefore, in order to prevent or at least minimize the occurrence of this type of malfunction encountered in the semiconductor devices, a stable backbias voltage having a small ripple is needed.